A first conventional optical head comprises an optical system in which a light spot is formed on an optical disc, and a light reflected from the optical disc is splitted to two lights, the first one of which is supplied to a first photodetector, and the second one of which is supplied via a cylindrical lens to a second photodetector, so that a tracking error is detected in the first photodetector by push-pull method, and a focusing error is detected in the second photodetector by astigmatism generated by the cylindrical lens.
In the first conventional optical head, the light spot is required to be small for high density recordation and reproduction of information. For this reason, a wavelength of a light radiated from a semiconductor laser is made small, and a numerical aperture NA of an objective lens for forming a light spot on the optical disc is made large.
In this first conventional optical head, however, there is a limitation to make a diameter of the light spot small. Especially, it is difficult for the light spot to be smaller than a diffraction limitation.
In order to make the diameter of the light spot smaller than the diffraction limitation, a second conventional optical head comprising a light shielding plate for shielding a central portion of a light beam radiated from a semiconductor laser as explained in the Japanese Patent Kokai Nos. 2-12623 and 2-12624.
In the second conventional optical head, however, there is a disadvantage in that side-lobes are generated to detect signals from adjacent pits, thereby generating noise to make it impossible in the worst case that high density recordation of information into and reproduction thereof from an optical disc is practically carried out.